1. Field of the Invention
The present invention relates to non-volatile memory devices and, more particularly, to methods of code programming read-only memory (ROM) semiconductor devices.
2. Description of Related Art
A non-volatile, semiconductor memory device is designed to securely hold data even when power is lost or removed from the memory device. The read-only memory (ROM) is a non-volatile memory device widely used in microprocessor-based digital electronic equipment for storing predetermined programs.
Arrays of memory cells are conventionally disposed in ROM devices for storing data, wherein each memory cell includes a transistor. These transistors, which typically comprise metal-oxide-semiconductor field effect transistors (MOSFETs), are disposed at intersecting bit lines and word lines of the memory device. Data bit values or codes held by these memory cell transistors are permanently stored in the physical or electrical properties of the individual memory cells. Generally speaking, a consequence of the non-volatile nature of a ROM is that data stored in the memory device can only be read.
The fixation of this “read-only” data into the ROM is performed during a code programming process at the original manufacture or fabrication of the memory device. Code programming a ROM typically entails ion implanting the read-only data into transistor channel regions of selected memory cells of the memory, thereby increasing the threshold voltage at which the MOSFET activates. The threshold voltage can be increased such that it is greater than the maximum possible applied voltage. This results in a permanently nonconductive or less conductive transistor, and thereby codes a binary “0” into the implanted MOSFET.
Since the channel regions of only selected memory-cell transistors are ion implanted, other areas of the memory device should be covered and protected during the ion-bombardment step. Accordingly, code photomasks have been developed in the prior art for permitting the implantation of ions only into selected regions of the semiconductor. Usage of code photomasks during the code programming process has lead to the characterization of these memory devices as mask ROMs.
Regarding code photomasks, these tools for facilitating code programming of the mask ROM operate using principles of photolithography. Photolithography is a method of transferring a pattern onto a substrate so as to create structures down to the scale of fractions of a micron. A photolithography process can be incorporated, for example, in the fabrication of many modern devices such as MEMS (micro-electro-mechanical systems), optics, and semiconductor devices including mask ROMs.
A typical optical photolithographic process is implemented by depositing onto a substrate such as a semiconductor wafer, by some means (usually a spinner), a layer of photosensitive resist which can be patterned by exposure to ultraviolet (UV) light or another radiation type. To undergo exposure, the photoresist covered wafer is placed beneath a photomask designed to prevent the penetration of radiation through certain portions of the photoresist. Predetermined areas of the photoresist then undergo a degree of polymerization or depolymerization, which can be a function of the nature and extent of photoresist exposure to the radiation. A chemical bath known as a developer can then be used to dissolve parts of the photoresist which remain depolymerized after the radiation by placing the wafer therein and allowing the wafer to be rinsed for a designated time period. Having received the pattern from the photomask, the layer of photoresist on the wafer is typically referred to as a layer of patterned photoresist.
A patterned photoresist layer can be created either on a bare wafer or on a number of previously generated layers of a wafer, with a limitation that the layer or layers should have somewhat planar surfaces to avoid problems including depth of focus variances. Common uses for patterned photoresist include selectively doping certain areas of a wafer while preventing other protected areas from being implanted, and selectively etching underlying layers on a substrate. When used as an implantation barrier, the patterned photoresist can prevent the underlying protected areas from receiving dopant, thereby allowing electrical properties of the substrate to differ between sites.
Code photomasks can be divided into the categories of pre-code masks and real-code masks. Pre-code masks provide dense identical patterns of openings, each of which defines a transistor. Real-code masks provide openings only for those transistors that are to be programmed.
In the practice of code programming ROMs, numerous methods exist by which the desired code can be implanted in the ROM. Two common coding methods both of which are utilized allegedly to minimize processing time and reduce the number of processing stages are used with equal aplomb. The first method involves forming a photoresist layer, and subsequently twice exposing the photoresist layer, once with the pre-code pattern and once with the real-code pattern. In this method a single photoresist plane is used for two exposures, making it difficult to control the overlay of the two exposures. As a result, an undesirable shifting of the implantation area may occur.
Another common coding method uses only a single mask (the real-code mask) for ion implantation, forgoing the pre-code mask. The real code is formed by a mask image on a single photoresist plane. This method requires a minimum of processing steps, however, since a pre-code mask is not utilized it can be difficult to control the actual sizes of the various processing windows (open areas). This can result in a reduced control over the ion dosage received by individual transistors at different locations on the ROM.
As an alternative solution which may avoid the aforementioned difficulties, prior-art photolithography approaches occasionally utilize an oxide layer in combination with one or more photoresist layers. Implementation of an oxide layer as the pre-code mask may achieve desired implanting goals in accordance with circuit fabrication objectives without many of the above-discussed problems. Known shortcomings, however, are presented in connection with fabrication processes utilizing oxide layers. For instance, further processing steps are required to pattern the oxide layers, which steps can lead to increased processing times, consumption of additional materials, and augmented costs. Undesirable particles can also be introduced during the oxide deposition and during the oxide patterning process. Furthermore, implementation of an oxide pre-code masking process may induce a critical dimension (CD) bias, and may cause etch uniformity related issues. Imprecise CD control during formation of a pre-code pattern in an oxide layer can adversely affect the real-code implantation process. In the context of mask ROM fabrication and coding, it is desirable to code program the memory devices as quickly and simply as possible, with a minimal expenditure of resources and a minimal risk of adverse particle introduction and CD bias.
A need thus exists in the prior-art for methods of manufacturing mask ROMs in which more accurate dosage control and processing windows can be obtained, while providing minimal processing times and materials, to thereby reduce defects and maintain a low cost. A need also exists for reliable code programming methods which can decrease the potential for particle contamination during the pre-code steps. Furthermore, with device sizes approaching the resolution limit of optical photolithography, wherein, for example, a code implantation area may be 0.15 um2, a need continues in the prior-art to exercise precise pre-code and real-code CD control to thereby maintain device performance in a cost effective manner.